How to fix modeling geometry/topology error after applying all modifiers?
Enter edit mode, select the whole mesh, in the menu choose &quot;mesh&quot; -&gt; &quot;symmetrize&quot;
Then select the correct &quot;Direction&quot; in the Redo panel (the pop-up panel with the settings for the tool)
Make sure your objects origin is in the centre of the object as this is where the symmetrize-ing will be based.
Blender docs: symmetrize